The present invention relates to an electrical filter and more particularly, to an electrical filter employing coaxial resonators, for example, transverse electro-magnetic mode coaxial resonators (referred to as TEM coaxial resonators herein) which have improved designs especially advantageous from the viewpoint of manufacture.
Generally, electrical filters utilizing coaxial resonators have been widely used in electrical and electronic equipment operating, for example, in VHF and UHF ranges.
Referring to FIG. 1, there is shown one example of the structure of a conventional coaxial resonator currently in production. In FIG. 1, the known 1/4 wavelength coaxial TEM resonator R generally comprises an inner conductor E.sub.1, outer conductor E.sub.2, and a dielectric member D of, for example, the titanium oxide group, filling the space between the inner conductor E.sub.1 and outer conductor E.sub.2. More specifically, the dielectric member D may have a cylindrical tube-like configuration having a thick wall, and, for example, silver paste which has a superior high frequency conductivity and good adhesion with respect to the dielectric material is applied to inner and outer wall surfaces of the dielectric member D by means of baking or like to form thereon the inner conductor E.sub.1 and outer conductor E.sub.2. Meanwhile, into the hollow interior of inner conductor E.sub.1, a central rod d, composed of a material similar to that of the tubular dielectric member D and having the same length as dielectric member D may be inserted and secured there.
The known 1/4 wavelength coaxial TEM resonator having the construction described above, however, has disadvantages with respect to connection with other electronic components, especially when it is to serve as part of a filter. More specifically, the open end of the resonator is normally connected to other components by means of capacitive coupling, and when capacitors are employed for such connections, it is necessary to take special measures for fixing the capacitors to the resonators. Moreover, it has been a general practice that the resonant frequency of a resonator of the above described type is adjusted by cutting the opposite faces of the resonator to alter its axial length, but such an adjustment cannot be efficiently made, particularly after the resonator is incorporated into a filter casing.
Meanwhile, regarding the electrical filters, there has been proposed, for example, in Japanese Open Patent Application No. 52-96844, an electrical filter employing coaxial TEM resonators as described in the foregoing mainly for the purpose of reducing the filter size, etc. in accord with the recent trend to miniaturization of electrical and electronic equipment, but the known electrical filter as described above still has some problems to be solved with respect to its performance, and with respect to the efficiency of its manufacture.